1. Field of the Invention
The present invention relates to digital audio coding systems that employ high frequency reconstruction (HFR) methods. It enables a more consistent core codec performance, and improved audio quality of the combined core codec and HFR system is achieved.
2. Description of Related Art
Audio source coding techniques can be divided into two classes: natural audio coding and speech coding. Natural audio coding is commonly used for music or arbitrary signals at medium bit rates. Speech codecs are basically limited to speech reproduction, but can on the other hand be used at very low bit rates. In both classes, the signal is generally separated into two major signal components, a spectral envelope and a corresponding residual signal. Codecs that make use of such a division exploit the fact that the spectral envelope can be coded much more efficiently than the residual. In systems where high-frequency reconstruction methods are used, no residual corresponding to the highband is transmitted. Instead, a highband is generated at the decoder side from the lowband covered by the core codec, and shaped to obtain the desired highband spectral envelope. In double-ended HFR systems, envelope data corresponding to the upper frequency range is transmitted, whereas in single-ended HFR systems the highband envelope is derived from the lowband. In either case, prior art audio codecs apply a time invariant crossover frequency between the core codec frequency range and the HER frequency range. Thus, at a given bit rate, the crossover frequency is selected such that a good trade-off between core codec introduced artifacts, and HER system introduced artifacts is achieved for typical program material. Clearly, such a static setting may be far from the optimum for a particular signal. The core codec is either overstressed, resulting in higher than necessary lowband artifacts, which inherent to the HER method also degrades the highband quality, or not used to its full potential, i.e., a larger than necessary HER frequency range is employed. Hence, the maximum performance of the joint coding system is only occasionally reached by prior art systems. Furthermore, the possibility to align the crossover to transitions between regions with disparate spectral properties, such as tonal and noise like regions, is not exploited.